The present invention relates generally to a substrate inspecting apparatus using electron beams, a substrate inspecting system and a substrate inspecting method, and more particularly to an inspecting apparatus, an inspecting system and an inspecting method which are suitable for inspecting defects on a semiconductor wafer and a photo mask.
With a higher integration of the semiconductor device, there has increasingly been enhanced a sensitivity required for detecting a defect and a foreign material on a semiconductor wafer and a photo mask. In general, a detection sensitivity under of a width of a pattern wire is needed for detecting the pattern defect and the foreign material which might cause a serious deterioration in terms of a quality of the product. Therefore, in the semiconductor wafer defect inspection with a design rule of 1/4 .mu.m or under, its pursue has come in recent years substantially to a limit of the pattern defect inspection based on an optical system. Such being the case, pattern defect inspecting apparatuses using electron beams have been developed as a substitute for the optical system inspection, which were disclosed in Japanese Patent Application Laid-Open Publication Nos.5-258703(1993) and 7-249393(1995).
Japanese Patent Application Laid-Open Publication No. 7-249393 discloses a method and an apparatus for detecting a pattern defect on a semiconductor wafer which involve the use of the electron beams. FIG. 1 shows the pattern defect detecting apparatus.
As illustrated in FIG. 1, a pattern defect detecting apparatus 80 comprises a primary optical system 81 including an electron gun, having a rectangular electron emission surface cathode, for irradiating a surface 85 of a sample 82 with electron beams 88 each taking a rectangular shape in section, and a quadrupole lens system. The apparatus 80 also comprise a secondary electron detecting system 84 including an electron beam detecting unit 86 for detecting secondary electrons, reflected electrons and backward scattered electrons 83 generated on the sample, and a mapping projection optical element for forming on the detecting unit 86 secondary/primary images of the secondary electrons, reflected electrons and backward scattered electrons 83, and further a detection signal processing circuit 87 for processing detection signals.
In the pattern defect detecting apparatus 80, the section of the electron beam falling upon the sample surface takes not circular and square shapes but the rectangular shape, thereby reducing a scan time. Besides, an aspect ratio of the rectangular electron beam is properly set, thereby controlling a density of the electric current of the electron beams. Further, the image signals detected by the electron beam detecting unit 86 are processed in parallel, and therefore this apparatus has such an advantage that the defect on the wafer pattern can be detected at a high speed.
In the defect inspection using the electron beams, in addition to the detection of a minute pattern defect on the wafer surface, it is feasible to detect even a defect on the semiconductor wafer, which is derived from an electric continuity defect (such as open- and short-circuit) by utilizing an image contrast (voltage contrast) occurred due to a difference in electric potential on the sample surface. The defect with an abnormality of the electric conduction inducing the above voltage contrast, is known as a voltage contrast defect. The principle for detecting this voltage contrast defect will be explained with reference to the drawings.
The sample surface is charged up upon the irradiation of the electron beams. A (positive/negative) polarity of the charging up at that time is determined based on a material of the surface of the semiconductor wafer as well as on an energy of the irradiated electrons. FIG. 2 is a sectional view schematically showing one example of the semiconductor wafer irradiated with the electron beams. When a semiconductor wafer 100 shown in FIG. 2 is irradiated with, e.g., electron beams 101 having a predetermined energy quantity, an electrically floating Al electrode pattern 92 on the wafer surface is charged up to the negative polarity. While on the other hand, when irradiated with the electron beams of which an acceleration voltage is on the order of 1 keV, a pattern 91 of an insulating layer composed of SiO.sub.2 etc. is charged up to the positive polarity. Further, if grounded as in the electrode pattern 93 even in the case of a pattern composed of the same material Aluminum (Al), the electron beams flow across the substrate, resulting in no charging up. Thus, the sample surface is charged up to the positive or negative polarity with a variety of values upon the irradiation of the electron beams, whereby the electric potential is induced on the sample surface. With this surface potential, there change an emission efficiency of the secondary electrons, the reflected electrons and the backward scattered electrons occurred on the sample, and a detection efficiency at which those electrons generated thereon are taken in by the detector. In the example shown in FIG. 2, there are a high emission efficiency and a high detection efficiency of the secondary electrons, reflected electrons and backward scattered electrons 102 occurred on the Al electrode pattern 92, and therefore a quantity of the detection signals by the electron detector is large. An electron emission efficiency of secondary electrons, reflected electrons and backward scattered electrons 103 occurred on the Al electrode pattern 93 becomes smaller due to the charging up than on the pattern 92. Hence, the quantity of the detection signals by the electron detector is smaller than in the case of the pattern 92. By contrast, on the insulating layer pattern 91, there are a large quantity of the secondary electrons, the reflected electrons and the backward scattered electrons which are absorbed to the substrate surface charged up to the positive polarity, and therefor the quantity of the detection signals is by far smaller than on other portions (the Al electrode patterns 92, 93).
Such a change in the detection signal quantity which is based on the change in the surface potential of the sample, appears as an image contrast of the electron image formed by the projection unit. This electron image contrast is termed a voltage contrast. Generally, the voltage contrast appears in the form of the charging up of the secondary electron image on a scanning electron microscope, or is utilized for an EB tester analysis.
The images of the secondary, reflected and backward scattered electrons, which are obtained by irradiating the surface of the semiconductor wafer with the electron beams, in variably contain the voltage contrast in addition to a contrast (configurational contrast) appearing depending on a configuration. Accordingly, if the electrical conduction defective portions such as the open- and short-circuits exist mutually in the interconnections and contact holes of the semiconductor interconnection pattern, the surface potentials at the defecting portions are different from the surface potential at the electrically normal portion, and consequently the voltage contrasts different from that at the normal portion appear.
For example, as in a semiconductor wafer 110 shown in FIG. 3, interconnection patterns 112, 113 composed of tungsten (W) will be explained. The interconnection patterns 112, 113 are each so designed as to be grounded and must have a decreased detection rate of the secondary electrons, the reflected electrons and the backward scattered electrons. As shown in FIG. 3, however, the contact hole is not sufficiently formed in the interconnection pattern 113, resulting in a state of the open-circuit. A large amount of the secondary electrons, the reflected electrons and the backward scattered electrons are thereby detected from the interconnection pattern 113, and it follows that the voltage contrast different from the electrically normal portion is detected in the detection image. The voltage contrast, of which an electric conduction condition is different from the normal portion, is hereinafter referred to as an abnormal voltage contrast.
Thus, the electric conduction defective portion on the semiconductor wafer can be inspected by detecting the abnormal voltage contrast.
There arises, however, a problem inherent in the prior art described above, wherein a detection accuracy of the abnormal voltage contrast declines for the following reason.
Namely, the surface potential of the voltage contrast defect is estimated to be under 10V, and hence it is required that a voltage contrast image corresponding to a difference in terms of the surface potential be used for detecting the above-described defect. Accordingly, when forming an image of the electrons having an energy width over the surface potential difference, an energy component exceeding the surface potential difference turns out to be a noise component, which might be a factor for deteriorating the detection accuracy of the abnormal voltage contrast. According to the prior art described above, the images of the electrons in all the energy ranges of the secondary electrons, the reflected electrons and the backward scattered electrons generated on the wafer serving as a sample, are formed on the detector, and hence it follows that the voltage contrast images obtained therefrom are formed of the electrons exhibiting the continuous energies. Therefore, the problem is that it is therefore difficult to detect a minute different in the surface potential on the wafer sample.
Further, in the prior art discussed above, it is unfeasible to quantitatively measure an electric characteristic of the voltage contrast defect from the obtained voltage contrast image.
As explained above, the defect electric characteristic such as the contact resistance value reflects in the surface potential of the wafer sample. Accordingly, if capable of quantitatively measuring the surface potential with respect to the voltage contrast image, the electric characteristic can be quantitatively measured, and its effect is extremely large.